Extreme events of stratospheric stationary waves and indications for stratosphere–troposphere coupling: simultaneous analysis in boreal winter
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Using monthly stratospheric geopotential height at 20 hPa derived from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis datasets, a planetary wave amplitude index (PWAI) is defined by wave numbers (WN) 1–3 over 55–75°N to indicate the strength of the stratospheric stationary waves. The vertical variability of the stratospheric stationary waves and their possible connection with the stratosphere–troposphere coupling have been investigated in the North Hemisphere winter [December–February (DJF)] for the period of 1950–2010. In terms of the stratospheric PWAI, a pair of bipolar extreme (strong and weak) stratospheric wave events is identified based on the top-ten principle. The comparisons of composite analysis for the bipolar events show that the stratospheric PWAI is an effective indicator for the dynamic coupling between the stratosphere and troposphere in the boreal winter. The results show that the opposite response in the stationary wave activity, atmospheric circulations, precipitation, and surface temperature is not only found in the stratosphere but also in the troposphere and surface. In the stratospheric top-ten extremely strong (strong10) events, the wave amplitude, poleward momentum, and heat fluxes in zonal WN1 tend to increase in the whole atmospheric layer from the stratosphere down to the surface. The polar vortex is enhanced in the stratosphere and reduced in the troposphere. Both the North Atlantic Oscillation (NAO) and North Pacific Oscillation (NPO) tend to a positive phase with the sea-level pressure (SLP) decreasing in the high latitudes and increasing in the mid–low latitudes. The precipitation tends to increase in the southern Asia and decrease in the southern Europe. The surface temperature becomes warmer in the middle of the Asian–European continent and cooler in southwest Asia and south Europe. In contrast, there is a clear opposing behavior except for a few small areas during the stratospheric top-ten extremely weak (weak10) events.
KeywordsWave Amplitude Geopotential Height North Atlantic Oscillation Middle Latitude Planetary Wave
The NCEP/NCAR monthly reanalysis data and precipitation are obtained from the NCAR web site. The long-term reconstructed surface temperature dataset is from the Fifth Coupled Model Intercomparison Project (CMIP5). The authors would like to thank these agencies and research groups for providing the data.
This work was supported by the National Oceanic and Atmospheric Administration (NOAA), National Environmental Satellite, Data, and Information Service (NESDIS), Center for Satellite Applications and Research (STAR). The views, opinions, and findings contained in this publication are those of the authors and should not be considered an official NOAA or US government position, policy, or decision.
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